EPROMs are well known types of read-only-memories that may be programmed electrically. A unique feature of an EPROM is that it retains data even with the power disconnected but can be erased by exposure to short wavelength ultraviolet light, and may be reprogrammed many times.
Typically, an EPROM cell has two conductive material gates: a floating gate and a control gate. The floating gate stores the information of the cell. The gates are typically made from doped polycrystalline silicon (polysilicon), metal, refractory metal silicide, etc. and combinations thereof.
Conventional EPROM cells use highly energetic carriers or "hot" carriers produced by a "drain avalanche field" to inject charge into the floating gate, as seen in FIG. 2. Shown in FIG. 2 is a schematic diagram of a conventional EPROM cell 10 with source region 12 and drain region 14 separated by a channel 16 within a semiconductor substrate 18. The control gate 22 is positioned over the floating gate 20 which has charge injected therein by the high electric field at the drain end of the channel 16.
Problems with conventional EPROM cells such as 10 in FIG. 2 include the conflict shown qualitatively in the chart of FIG. 3 in which log I.sub.sub and log I.sub.s are plotted as functions of V.sub.g. The substrate current, I.sub.sub, and the gate current, I.sub.g, peak at the lower and higher values of gate voltage, V.sub.g, respectively as shown. V.sub.g and V.sub.d are at odds to control the vertical electric field, therefore high V.sub.g is needed to obtain high I.sub.g. To get high vertical electric field in the correct direction for electron injection, we need high V.sub.g. Additionally, however, high V.sub.d is required to produce impact ionization "Impact ionization" is a term which refers to the process by which a high electric field produces highly energetic or "hot" electrons and holes. To get a large lateral electric field in the channel, V.sub.g needs to be approximately equal to (V.sub.d /2), and that actually, I.sub.g peaks for a V.sub.g greater than (V.sub.d /2). Both of these conditions are needed to produce high gate current. As a result, high operating voltages are needed, and programming is slow on large memory chips. These characteristics are contradictory to the desired characteristics of EPROM memory chips, which include low programming voltages and fast programming; also desired for easy testing of EPROM chips.